First evidence of sex differences in the duration of avian embryonic period: consequences for sibling competition in sexually dimorphic birds

Parental favoritism in birds would be enhanced if parents cancontrol any egg feature influencing the ontogeny of the embryoduring incubation. Egg size and composition may influence theduration of incubation and hatching periods, and eggs bearingembryos of different sex may differ in size and composition.Therefore, the sex of the embryo could also influence its ontogenybefore hatching. We tested this prediction by investigatingthe duration of the embryonic period of different-sex embryosin the Eurasian kestrel (Falco tinnunculus), a sexually dimorphicraptor in which adult females are approximately 20% heavierthan are adult males. We found the first evidence of sex differencesin the duration of the embryonic period in avian eggs. Femaleembryos had a shorter embryonic period than did male embryos,which allowed females to hatch earlier in the hatching sequenceand assume a higher rank than that of males in the intrabroodsize hierarchy. Embryos with a fast growth and development resultedin hatchlings with greater residual reserves and thus largermass, which suggests that a shorter embryonic period requiresless maintenance metabolism relative to growth. Our resultsalso indicated that early hatching may be advantageous to gaina high rank in the size hierarchy within the brood independentlyof the effect of sex on fledgling mass. Sex differences in avianegg ontogeny may therefore be a factor shaping life-historytraits associated with parental control of sibling competition,which should be addressed in any future work on optimal reproductiveinvestment.